Microelectromechanical systems (“MEMS,” hereinafter “MEMS devices”) are used in a wide variety of applications. For example, MEMS devices currently are implemented as microphones to convert audible signals to electrical signals, as gyroscopes to detect pitch angles of airplanes, and as accelerometers to selectively deploy air bags in automobiles. In simplified terms, such MEMS devices typically have a movable structure suspended from a substrate, and associated circuitry that both senses movement of the suspended structure and delivers the sensed movement data to one or more external devices (e.g., an external computer). The external device processes the sensed data to calculate the property being measured (e.g., pitch angle or acceleration).
MEMS microphones are being increasingly used in a greater number of applications. For example, MEMS microphones are often used in cellular phones and other such devices. To penetrate more markets, however, it is important to obtain satisfactory sensitivity and signal to noise ratios that match more traditional microphones.
MEMS microphones typically include a thin diaphragm electrode and a fixed sensing electrode that is positioned alongside the diaphragm electrode. The diaphragm electrode and the fixed sensing electrode act like plates of a variable capacitor. During operation of the microphone, charges are placed on the diaphragm electrode and the fixed sensing electrode. As the diaphragm electrode vibrates in response to sound waves, the change in distance between the diaphragm electrode and the fixed sensing electrode results in capacitance changes that correspond to the sound waves. These changes in capacitance therefore produce an electronic signal that is representative of the sound waves. Eventually, this electronic signal may be processed to reproduce the sound waves, for example, on a speaker.
FIG. 1 shows the general structure of a micromachined microphone as known in the art. Among other things, the micromachined microphone includes a diaphragm 102 and a bridge electrode (i.e. backplate) 104. The diaphragm 102 and the backplate 104 act as electrodes for a capacitive circuit. As shown, the backplate 104 may be perforated to allow sound waves to reach the diaphragm 102. Alternatively or additionally, sound waves can be made to reach the diaphragm through other channels. In any case, sound waves cause the diaphragm to vibrate, and the vibrations can be sensed as changes in capacitance between the diaphragm 102 and the bridge 104. The micromachined microphone typically includes a substantial cavity 106 behind the diaphragm 102 in order to allow the diaphragm 102 to move freely.
Many MEMS microphones use a diaphragm that is anchored completely around its periphery, similar to the head of a drum. Such diaphragms can present a number of problems. For example, in the presence of sound waves, such diaphragms tend to bow rather than move up and down uniformly, as shown in FIG. 2A. Such bowing can negatively affect the sensitivity of the microphone, specifically due to the limited displacement of the diaphragm causes by internal tension and the variation in distance between portions of the diaphragm and the fixed sensing electrode. Also, such diaphragms can suffer from sensitivity to stresses (e.g., heat expansion), which can distort the shape of the diaphragm and can affect the mechanical integrity of the diaphragm as well as the sound quality produced by the microphone.
Some MEMS microphones have a diaphragm that is movably connected with its underlying stationary member (referred to hereinafter as a “carrier”) by way of a plurality of springs. The springs tend to enable the diaphragm to move up and down uniformly (i.e., like a plunger), as shown in FIG. 2B.